The present invention relates to a method of manufacturing insulated gate thin film field effect transistors (referred to hereinafter simply as thin film transistors), whereby a number of thin film transistors may be formed simultaneously upon a substrate which can be selected from a wide range of materials, including glass.
At the present time, there is an increasing requirement for display devices having a high density of display elements, which can be manufactured as economically as possible. The majority of such display devices used in miniature electronic devices, such as electronic wristwatches, are of the liquid crystal type, and the highest display density is obtainable by using a matrix display configuration, in which an individual switching element is provided for each display element, these switching elements being positioned at the intersections of row and column electrodes and controlled by drive signals applied thereto, with the switching elements generally comprising insulated gate thin film field effect transistors. The performance and reliability of such a display is greatly affected by the electrical characteristics, such as switching speed etc, of these thin film transistors. It is desirable that these thin film transistors be formed upon a transparent substrate in such a display device, and in the case of a display device of transmission type (i.e. in which light is transmitted through the display device from the rear toward the viewer), such as might be used in a television display using such a device, it is essential that the thin film transistors be formed on a transparent substrate, for example one of the substrates of a liquid crystal display. The semiconductor thin film used in such thin film transistors generally comprises a film of polycrystalline silicon, deposited on a substrate by, for example, chemical vapor deposition. Such a semiconductor film has a very high electrical resistivity. In the prior art, it has been possible to form thin films of oxide, for use as the gate insulating layer of field effect transistors upon a base layer which has fairly high electrical conductivity, such as a layer of bulk semiconductor. An oxide film formed upon such a base layer can be made extremely uniform, by deposition using the anodic oxidation process. However it has not hitherto been possible to use anodic oxidation to form an oxide film for use as the gate insulating layer of thin film field effect transistors upon a base layer having high resistivity, such as a thin semiconductor film. In the prior art, the only method which has been available for forming the gate insulating layer of such thin film transistors has been to either perform thermal oxidation of the polycrystalline silicon film at an extremely high temperature, to form a film of SiO.sub.2, or to form a film of an insulating material such as Al.sub.2 O.sub.3, by a method such as chemical vapor deposition or physical vapor deposition. The thermal oxidation method provides an oxide film which is extremely uniform in thickness and free from internal defects. As a result, the thin film transistors produced by such a method can have excellent switching and other electrical characteristics. However, as a result of the high temperature processing required to form the oxide film for use as a gate insulating layer of the thin film transistors, only a very limited range of materials are available for use as the substrate for such transistors, and it is not possible to use an inexpensive transparent material such as glass. If deposition of a gate insulating layer by chemical or physical vapor deposition is performed, then the processing temperature is not excessive, so that it is possible to use a substrate material which is not capable of withstanding high temperatures, such as glass. However the gate insulating layer which is formed by such a method is greatly inferior to the oxide film produced by the thermal oxidation method, so that it is not possible to produce thin film transistors having good electrical characteristics by such a prior art method.
As described hereinafter, the method of manufacturing thin film transistors according to the present invention overcomes the disadvantages of prior art methods, as described above, and enables a number of thin film transistors having excellent electrical characteristics to be formed upon a substrate which may consist of an inexpensive transparent material such as glass, so that the present invention is highly suitable for manufacture of thin film transistors for use as switching elements in a high-density matrix display device.